It is generally desirable to increase as much as practical the amount of data which can be stored on an information storage disc. Data is generally stored on a disc on concentric circular tracks. Obviously, it is desirable to fit as many tracks as possible on a disc surface, which means that the tracks should be as narrow as practical and as close together as practical. However, it is also necessary to position a read/write head over the middle of any desired track and to keep it there during rotation of the disc while the data is written into the track or read from the track. As the tracks become ever smaller in width and are spaced closer and closer together, it becomes increasingly difficult to correctly position the read/write head. It is this ability to reliably position a read/write head over the center of a desired track while the disc is rotating, which limits the practical density of the recording discs.
With magnetic storage discs, typically an open loop positioning system with no feedback control is used to coarse position either one or an array of read/write transducers over a desired track. With flexible magnetic storage discs, this is typically the only positioning system for the read/write head. With hard magnetic storage discs, coarse positioning is generally followed by fine positioning using a positioning system with feedback control to precisely position the read/write head over the center of the desired track. The fine positioning system generally locks onto and tracks a magnetic pattern of some kind. Most of these magnetic servo schemes use a significant portion of the recording surface for a magnetic servo pattern. This obviously reduces the amount of recording surface available for recording data.
An objective of this invention is to reduce the access time to a given target track without the use of a closed loop servo system. More particularly, an objective is to provide a disc drive with low access times without sacrificing disc surface space to servo data storage.
More particularly, it is an objective herein to provide a simply implemented system which can reduce the time necessary to finely position a read/write head over the center of the target track. This reduction in access time is very important in reducing the time necessary to move the read/ write head from one track to another over a disc surface.
In the past, it has been identified as being highly desirable in fine positioning a read/write head to reduce the settling time; that is, the time necessary to slow down and then finally stop a read/write head in its movement from one track to another. In order to achieve a reduction from about 65 milliseconds access time to below 40 milliseconds, open loop control methods were initially used wherein a minimum ring-out time was achieved by correctly timing the step input command pulses to a stepper motor. This open loop approach, however, has limited effectiveness because motor parameters will vary due to manufacturing tolerances. Therefore, with an invariant algorithm, the step response will no longer have minimum ring-out for every motor. The principles of closed loop methods to minimize the single step response ring-out time, that is, the time necessary to finally settle the read/write head over the target track, have been described in the literature, but they require an external sensor or an extensive amount of electronics to decode winding voltages and produce a feedback signal.
Therefore, it is an objective of this invention to use and incorporate a simple voltage feedback method that uses the fact that the motor step size is small compared to one electrical cycle of the back emf of the motor.